A microgrid is a semiautonomous grouping of distributed energy resources (distributed generation and energy storage) and loads within a local area. The loads can be one utility “customer,” a grouping of several sites, or dispersed sites that operate in a coordinated fashion. The distributed electric generators can include reciprocating engine generators, microturbines, fuel cells, photovoltaic/solar and other small-scale renewable generators. All controllable distributed energy resources and loads are interconnected in a manner that enables devices to perform certain microgrid control functions. For example, the energy balance of the system must be maintained by dispatch and non-critical loads might be curtailed or shed during times of energy shortfall or high operating costs. While capable of operating independently of the macrogrid (in island mode), the microgrid usually functions interconnected (in grid-connected mode) with a sub-station or grid (i.e. macrogrid), purchasing energy from the macrogrid and potentially selling back energy and ancillary services at different times. Microgrids are typically designed based on the total system energy requirements of the microgrid. Heterogeneous levels of power quality and reliability are typically provisioned to end-uses. A microgrid is typically presented to a macrogrid as a single controllable entity.
Most microgrid control systems adopt either a centralized or distributed mechanism. Distributed microgrid control systems are mostly used in remote area islanded and weakly grid-connected microgrids, in which system stability is a major concern and the control objective is mainly to maintain the microgrid dynamic stability. Centralized microgrid control systems perform the coordinated management of the microgrid in a central controller, which monitors overall system operating conditions, makes optimal control decisions in terms of minimizing operation cost, reduces fossil fuel consumption, provides services for utility grid, etc. and then communicates power set points to distributed energy resources and control commands to controllable loads within the microgrid. Most conventional centralized microgrid control systems implement either a so-called ‘day-ahead’ DER (distributed energy resource) scheduling process combined with online economic dispatch (ED), or online-ED, across multiple time intervals. These solutions attempt to provide an optimized operation strategy over a predefined period of time while account for the renewable generation and load forecast.
The day-ahead DER scheduling with online ED approach generates an optimal operation plan for the next 24 hour period based on the day-ahead renewable generation and load forecast for the microgrid, and the ED is executed in real time the next day using the day-ahead DER schedule results. Due to imprecise forecasting techniques and high variability in renewable generation and load demand, the DER schedule executed in the day-ahead time frame cannot provide reliable operation planning and therefore adversely affects the online ED.
Online ED over multiple intervals incorporates the most recent generation and load forecast into the operation decision. However, this approach has considerable computation complexity at each execution interval (e.g., every 5 to 15 minutes) in real time to provide the control decision not only for the current interval, but also for future intervals. Due to the heavy computation burden, a simplified optimization, which only considers the power balance of the microgrid, is usually deployed instead of more detailed operating constraints provided by power flow analysis.